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post #1 of (permalink) Old 02-11-2002, 12:15 PM
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Ported vacumn

Hi Guys,
Does anyone know what the difference between "ported" and non "ported vacumn is? Is it just that one is from the carbuerator and the other is from the manifold? I've never come across an explanation of this.

By the way; my engine, post TR upgrade is running great! What a difference!

thanks,
Chris

'84 CJ7

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post #2 of (permalink) Old 02-11-2002, 03:39 PM
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Re: Ported vacumn

It was my understanding that manifold vaccuum has vaccuum all the time, whereas ported only has vaccuum when you step on the gas.

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post #3 of (permalink) Old 02-11-2002, 03:57 PM
 
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Re: Ported vacumn

Manifold vacuum is taken from, well, the manifold. Which means that when the motor is running, the less throttle you give it, the more vacuum is created in the manifold by the engine. As you open the throttle, there is less restriction to air flow into the manifold, and vacuum drops off. In some scenario's, like driving in a given gear, less vacuum means more load on the engine. Power brakes and tvac (the heat gate controlling warm air into your air cleaner) also run off manifold vacuum. They lose their power source, vacuum, at wot. Then again, who needs brakes at wot?

Ported vacuum is taken from within the carb's venturi, above the throttle plates. As you open the throttle, it rises quickly, as manifold vacuum drops, then eventually as wide open throttle (wot), both drop off. Most stock distributor's vacuum advance run off ported vacuum. More throttle, more advance, to a point. Some are hooked up to a combo of the two depending on how warm the engine is. You tune your idle circuit of your carb by watching manifold vacuum, as the higher the vacuum, the stronger the motor is running in proportion to the small amount the throttle is open for idle operation. Hope this helps.

tim

84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster
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post #4 of (permalink) Old 02-11-2002, 04:18 PM
 
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Re: Ported vacumn

see if this helps you decide which vacuum source to use

http://www.mame.mu.oz.au/salman/vac-adv-2
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post #5 of (permalink) Old 02-11-2002, 07:32 PM
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Re: Ported vacumn

The underhood sticker on my 84 258 says to use manifold vacuum for distributor advance. Can anyone confirm this? I'm pretty sure I read it right because it also says to disconnect the advance hose when timing the engine.

TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition, TR Ignition Upgrade
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post #6 of (permalink) Old 02-11-2002, 08:10 PM
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Re: Ported vacumn

Texas1al- I also run the vac adv off manifold vacuum on a 1988 258 with BBD . There had been some postings approx a year ago here discussing the ported vs. manifold vac for the Vac adv. It was my understanding that though some folks have made a conscious decision to use ported vac for vac adv , there are some engines with OEM designed with manifold vac to vac adv . If I remember correctly ,there had been some vac schematics which I had obtained from library auto books which showed ported vac to 258 engines vac adv . These misled me for a while . I have settled and feel comfortable with man vac to my '88 258 with BBD based upon finding more precise schematics in the forums. This was an area of concern for me a while ago. My conclusion is that you need to find a schematic which matches your engine . I am not an expert . Hope you are as lucky as I in finding the correct schematic. The underhood sticker sounds authoritative.

post #7 of (permalink) Old 02-11-2002, 09:20 PM
 
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Re: Ported vacumn

Tim is as close as anyone I've seen post here!

Manifold vacuum is sourced just like it says, from the manifold (anything below the throttle blades is manifold vacuum).
Manifold vacuum is dependant on engine rpm, and throttle blade opening. (blades open and let more air into the vacuum, the vacuum is naturally less with larger throttle openings...)

SPARK PORTED VACUUM is a little different.
If fact, the only similarities between spark ported vacuum and manifold vacuum is the two are vacuum sources...
Spark Ported Vacuum is taken from between the throttle blades and the Venturi 'throat'.

Manifold vacuum will increase anytime the throttle blades are closed, and instantly decrease anytime the throttle blades are opened.

Ported vacuum will be dependant on the angle or opening of the throttle blades, and how much air is passing through the Venturi.

When you throw open the throttle blades, the manifold vacuum will fall to nearly nothing.
If you have your vacuum advance plugged into manifold vacuum, not only will you loose all of your manifold vacuum, port & manifold velocity, but you will loose all of your vacuum advance.
Some companies use a vacuum rate retard in the vacuum line. This little gadget helps smooth out the large swings of the manifold vacuum when you change throttle position, but it's only a patch, and can't affect the same control as a ported vacuum source.

Spark Ported Vacuum rises as air flow through the Venturi body rises...
With ported vacuum, you will not loose all of your vacuum advance when the throttle blades are opened, and the vacuum will increase as the rpm rises, allowing for more vacuum advance.

This may be nit picking, but you can better control the vacuum advance, and the ignition control it has with ported vacuum.
This is an old racer trick to use the spark ported vacuum, and a weaker spring in the vacuum advance instead of a heavy spring in the vacuum advance, and all kinds of gadgets to control the vacuum to the ignition controls.


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post #8 of (permalink) Old 02-11-2002, 09:41 PM
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Re: Ported vacumn

TR,

Not to beat this one to death, but why would the OEM specs specify manifold vac to advance the timing? What you and others have said about ported vac makes perfect sense to me and seems to be the only way to get increasing levels of ignition advance as RPM increases. With manifold vac, the timing at higher RPM must be retarded versus idle, this doesn't make any sense. Should I hook to ported vac on my Carter bbd? If I do, I would assume that I can leave the hose hooked up while adjusting the timing. What would be a good fast idle timing degree with ported vac?

TEXAS1AL

84 CJ-7; 258 I6; Restored-to-Stock Condition, TR Ignition Upgrade
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post #9 of (permalink) Old 02-11-2002, 10:25 PM
 
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Re: Ported vacumn

I have dual vacume gauges mounted in front of my steering wheel (yes I have no life)and watch them when I am bored LOL .after your ported vac. kicks in around 1000 RPM they both run in synch,even as you give it gas ,rising and falling with your use of the go juice pedal,untill you take your foot off the pedal manifold vac jumps to appx 25 " and ported goes to 0"..To drop your manifold vac to 0 " you have to have your foot deep into the gas peddel then it evens out anyhow to around 15 " while you are crusing around 40 MPH
The TSM claims manifold vac is used to about 140 * when the CTO kicks in and switches to ported vac this is said to improve cold weather drivability.which is when my engine starts to knock.But that is another post LOL
The 1980 TSM has about 5 or 6 vacume diagrams for just the 6 cylinde engine involving several cto's and other gizmos ,heavy duty cooling system etc.

Ray
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post #10 of (permalink) Old 02-11-2002, 11:25 PM
 
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Re: Ported vacumn

To tell the truth, I have no idea why they use manifold vacuum.
Spark ported vacuum source was developed and designed into the system to do one thing, operate the vacuum advance.

This is just speculation, but...
Maybe it was the way to achieve some emissions goal.
Maybe it was a patent thing at one time.
Maybe it was a designer sh*t fight between the companies at one time.

I have no idea, but I suspect it was emissions and maybe patent issues.
(my money is on emissions controls, wanting to retard when the throttle made any large openings)

I do know in test after test on the dyno, we (and everyone from Holley to Edelbrock) have proven the spark ported vacuum over the manifold vacuum for power, economy and reliability.
If every carb designer didn't believe in spark ported vacuum, they wouldn't make provisions for it.

All chevy carbs used on years with vacuum advance canisters had spark ported vacuum.
Almost all Motorcraft carbs had spark ported or limited vacuum ports (some times called 'ratio' vacuum ports).
Chrysler hasn't used a vacuum advance on the distributor for years, but they still had a ported vacuum source on the carbs...

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